Radio transmission apparatus and radio transmission method

ABSTRACT

Data streams stored in buffers are modulated by modulation sections. Multipliers multiply the signals output from the modulation sections by weights output from a weight control section. The signals output from the multipliers are added up by addition sections, subjected to radio transmission processing by transmission radio sections and sent through antennas. A buffer control section controls the buffers based on a retransmission count output from a retransmission count detection section. The weight control section outputs weights different from weights at the time of previous transmission to the multipliers every time data is retransmitted. This allows a diversity gain at the time of data retransmission to be increased even if a time variation of the propagation path environment for radio signals is slow.

CROSS-REFERENCE(S) TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.12/490,231 filed Jun. 23, 2009, which is a continuation of U.S.application Ser. No. 10/498,052 filed Jun. 9, 2004, which is a nationalphase under 35 USC 371 of PCT/JP2003/10200 filed Aug. 11, 2003, which isbased on Japanese Patent Application No. 2002/268968 filed Sep. 13,2002, the entire contents of each of which are incorporated by referenceherein.

BACKGROUND

1. Technical Field

The present invention relates to a radio transmission apparatus andradio transmission method.

2. Description of the Related Art

As a technology for realizing communications of large-volume data suchas images, MIMO (Multi-Input Multi-Output) communications are beingstudied actively in recent years.

Among them, BLAST (Bell Laboratories Layered Space-Time) in particularis a focus of attention as an application capable of realizinghigh-speed transmission using an MIMO channel. This is a technique fortransmitting mutually independent (or coded) streams from a plurality oftransmission antennas and detecting the respective streams whilerepeating spatial filtering and removal of replicas on the receivingside.

Furthermore, when the MIMO channel information is known to thetransmitting side, it is known that a greater channel capacity can beobtained. More specifically, this is realized by carrying outdirectivity control using an eigen vector obtained through singularvalue decomposition of a matrix which consists of the respective channelresponses of the MIMO channels as elements and forming a spatiallyorthogonal channel (eigen channel). That is, when the MIMO channelinformation is known to the transmitting side, it is possible to form anorthogonal channel through multi-beam formation using the eigen vector,performing transmit power control through an irrigation theorem andthereby maximize the channel capacity (e.g., TECHNICAL REPORT OF IEICERCS2002-53 (2002-05), Institute of Electronics, Information andCommunication Engineers).

When the above described technology is applied to an actual apparatus,radio transmission is carried out after preparing a plurality oftransmission systems capable of carrying out transmission processing ona plurality of transmission data streams and assigning weights bymultiplying the transmission signals by their respective complex weights(hereinafter simply referred to as “weights”).

When a bit error rate on the receiving side does not satisfy apredetermined value, an automatic retransmission request (ARQ; AutomaticRepeat reQuest) is also generally practiced whereby the receiving sidesends a retransmission request signal to the transmitting side and thetransmitting side retransmits the same transmission data in response tothis request.

Especially packet transmission which transmits data traffic is requiredto guarantee error-free data transmissions and therefore error controlthrough ARQ is indispensable. In addition, when an adaptive modulationand error correction intended to improve the throughput by selecting anoptimal modulation system or coding system according to the condition ofa propagation path (path) are applied to packet transmission, it is notpossible to avoid measuring errors or packet errors caused by a controldelay, etc., and therefore the 3GPP (3rd Generation Partnership Project)also standardizes the use of a hybrid ARQ (hereinafter referred to as“HARQ”) which incorporates an FEC (Forward Error Correction) function.

Therefore, by carrying out an MIMO communication using a plurality ofantennas during data transmission to realize a large-volume datacommunication and retransmitting data when received data contains errorson the receiving side and by combining received data at the time ofinitial transmission with the received data at the time ofretransmission using HARQ on the receiving side, a considerableimprovement of throughput can be expected for this radio communicationsystem.

However, even when received data contains errors and data isretransmitted, if a time variation of an environment of a propagationpath which a transmission signal follows is slow (see FIG. 1), forexample, when the communication apparatus is at rest or moving at a lowspeed, the diversity gain obtained with reception power on the receivingside is small, and therefore there is a problem that the throughput ofthe radio communication system hardly improves even if data isretransmitted.

This is because when the time variation of the propagation pathenvironment is slow, a signal whose reception level is low at the timeof initial transmission also has a low transmission level at the time ofdata retransmission, and therefore data cannot be demodulated correctlyeven if the data at the time of initial transmission and data at thetime of retransmission are combined. Furthermore, when a multi-antennatechnology such as MIMO or STC (Space-Time Coding) is used, if the timevariation of the propagation path environment is slow, there is littlevariation in the fading condition between initial transmission and dataretransmission, and the combined data cannot be demodulated correctly.

BRIEF SUMMARY

It is an object of the present invention to increase a diversity gainobtained through data retransmission when data retransmission iscontrolled by a radio communication system (e.g., adaptive array antennatechnology, MIMO technology, STC technology, etc.) which transmits aplurality of data streams using a plurality of antennas.

This object is attained through a radio transmission apparatus and aradio transmission method which artificially changes the environment ofa propagation path which data streams follow after the transmission fromthat at the time of previous transmission.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 illustrates reception power on a receiving side when a timevariation of an environment of a propagation path which a transmissionsignal follows is slow;

FIG. 2 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 1 of the presentinvention;

FIG. 3 is a block diagram showing a configuration of a radio receptionapparatus according to Embodiment 1 of the present invention;

FIG. 4 is a sequence diagram showing a flow of radio communicationaccording to Embodiment 1 of the present invention;

FIG. 5A illustrates weights multiplied on transmission signals by aweight control section;

FIG. 5B illustrates weights multiplied on transmission signals by theweight control section;

FIG. 6 is a conceptual view illustrating directivity of a transmissionsignal;

FIG. 7A illustrates quality on the receiving side of a signaltransmitted from a conventional radio transmission apparatus;

FIG. 7B illustrates quality on the receiving side of a signaltransmitted from the conventional radio transmission apparatus;

FIG. 8A illustrates quality on the receiving side of a signaltransmitted from the radio transmission apparatus according toEmbodiment 1 of the present invention;

FIG. 8B illustrates quality on the receiving side of a signaltransmitted from the radio transmission apparatus according toEmbodiment 1 of the present invention;

FIG. 9 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 2 of the presentinvention;

FIG. 10A illustrates a delay time of a transmission signal for eachtransmission system;

FIG. 10B illustrates a delay time of a transmission signal for eachtransmission system;

FIG. 11 illustrates transmission timings of a transmission signal;

FIG. 12A illustrates reception power of a conventional apparatus;

FIG. 12B illustrates reception power of the conventional apparatus;

FIG. 12C illustrates reception power of the conventional apparatus;

FIG. 13A illustrates reception power according to Embodiment 2 of thepresent invention;

FIG. 13B illustrates reception power according to Embodiment 2 of thepresent invention; and

FIG. 13C illustrates reception power according to Embodiment 2 of thepresent invention.

DETAILED DESCRIPTION

The present invention assumes roughly two cases for a method of changinga propagation path environment of a transmission signal. A first case isa method of changing weights to be multiplied on the transmission signaland a second case is a method of changing timings for transmitting thetransmission signal.

With reference now to the attached drawings, embodiments of the presentinvention will be explained in detail below. Note that Embodiment 1describes a case where a weight to be multiplied on a transmissionsignal is changed and Embodiment 2 describes a case where a timing fortransmitting a transmission signal is changed.

Embodiment 1

FIG. 2 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 1 of the presentinvention. Here, a case where two streams; data stream #A and datastream #B are transmitted using two antennas will be explained as anexample.

The radio transmission apparatus shown in FIG. 2 is provided withbuffers 101, modulation sections 102, multipliers 103, addition sections104, transmission radio (RE) sections 105, transmission antennas 106, aretransmission count detection section 110, a buffer control section 111and a weight control section 112.

In FIG. 2, the data stream #A is input to the buffer 101-1 and the datastream #B is input to the buffer 101-2.

The buffers 101-1 and 101-2 store the input data streams in preparationfor a data retransmission request from a radio reception apparatus.Then, upon reception of an instruction for data transmission from thebuffer control section 111, the buffers 101-1 and 101-2 output the datato the modulation sections 102-1 and 102-2.

The modulation section 102-1 carries out modulation processing on thedata stream output from the buffer 101-1 and outputs the data stream.The signal output from the modulation section 102-1 is branched at somemidpoint and output to the multipliers 103-1 and 103-2. Likewise, themodulation section 102-2 carries out modulation processing on the datastream output from the buffer 101-2 and outputs the data stream. Thesignal output from the modulation section 102-2 is branched at somemidpoint and output to the multipliers 103-3 and 103-4.

The multiplier 103-1 multiplies the signal output from the modulationsection 102-1 by a weight output from the weight control section 112 andoutputs the weighted signal to addition section 104-1. On the otherhand, the multiplier 103-2 multiplies the signal output from themodulation section 102-1 by a weight output from the weight controlsection 112 and outputs the weighted signal to addition section 104-2.

Likewise, the multiplier 103-3 multiplies the signal output from themodulation section 102-2 by a weight output from the weight controlsection 112 and outputs the weighted signal to addition section 104-1.On the other hand, the multiplier 103-4 multiplies the signal outputfrom the modulation section 102-2 by a weight output from the weightcontrol section 112 and outputs the weighted signal to addition section104-2. The weights multiplied on the signals by the multipliers 103-1 to103-4 will be described in more detail later.

The addition section 104-1 adds up the weighted signals output from themultipliers 103-1 and 103-3 and outputs the addition result to thetransmission radio section 105-1. Likewise, the addition section 104-2adds up the weighted signals output from the multipliers 103-2 and 103-4and outputs the addition result to the transmission radio section 105-2.

The transmission radio section 105-1 carries out predetermined radiotransmission processing such as up-conversion on the signal output fromthe addition section 104-1, converts the signal to a radio signal andsends the radio signal through the antenna 106-1. On the other hand, thetransmission radio section 105-2 likewise carries out radio transmissionprocessing on the signal output from the addition section 104-2 andsends the signal through the antenna 106-2.

The radio reception apparatus which has received the signals sent fromthe antennas 106-1 and 106-2 carries out error detection on the receivedsignals and sends a NACK signal to the radio transmission apparatusaccording to this embodiment when an error is detected or sends an ACKsignal when no error is detected.

The retransmission count detection section 110 detects a retransmissioncount of the data from the ACK/NACK signal notified from the abovedescribed radio reception apparatus and outputs the retransmission countto the buffer control section 111.

The buffer control section 111 outputs control signals of data output tothe buffers 101-1 and 101-2 based on the retransmission count outputfrom the retransmission count detection section 110. More specifically,when the radio reception apparatus sends a data retransmission request,the buffer control section 111 controls the buffers 101-1 and 101-2 soas to output the data stored at the time of the previous transmissionagain.

The weight control section 112 includes a table storing a plurality oftypes of weights, selects a weight to be multiplied on a transmissionsignal from the table according to the retransmission count detected bythe retransmission count detection section 110 and outputs the weight tothe multiplier 103. When data is retransmitted, it references the weighttable again and outputs weights different from those at the time ofinitial transmission to the multipliers 103-1 to 103-4.

FIG. 3 is a block diagram showing a configuration of a radio receptionapparatus according to this embodiment.

This radio reception apparatus is provided with reception antennas 151,reception radio (RF) sections 152, a MIMO reception section 153, buffers154, demodulation sections 155, error detection sections 156, ACK/NACKsignal generation sections 157 and retransmission count detectionsections 158.

In FIG. 3, the reception radio (RF) sections 152 carry out predeterminedradio processing such as down-conversion on signals received through thereception antennas 151 and outputs the signals to the MIMO receptionsection 153.

The MIMO reception section 153 separates the signals output from thereception radio sections 152 into two substreams, that is, streams #Aand #B (MIMO reception processing) using propagation path characteristicinformation and outputs the streams to the respective buffers 154. ThisMIMO reception processing causes an inverse matrix of a matrix of 2rows×2 columns consisting of characteristics of propagation paths whichthe respective signals sent from the two antennas on the transmittingside follow as elements to act on the received signals to thereby obtaintwo substreams.

In the case of packet data at the time of initial transmission, thebuffers 154 immediately output this data to the demodulation sections155. When a retransmission packet is sent, this data is temporarilystored and demodulated. When the packet is received correctly and an ACKsignal is returned, the buffers are cleared. Being notified of theretransmission count from the retransmission count detection section158, the buffer 154 can decide whether the packet is sent at the time ofinitial transmission or at the time of retransmission.

The demodulation sections 155 carry out demodulation processing on thedata streams output from the buffers 154 and obtain data stream #A anddata stream #B.

The error detection sections 156 detect errors and notify to theACK/NACK signal generation sections 157 of the errors.

When notified from the error detection section 156 that an error hasbeen detected, the ACK/NACK signal generation sections 157 generate aNACK signal and send the NACK signal to the radio transmission apparatusand retransmission count detection section 158 or when notified from theerror detection sections 156 that no error has been detected, theACK/NACK signal generation sections 157 send an ACK signal to the radiotransmission apparatus and retransmission count detection section 158.

FIG. 4 is a sequence diagram showing a flow of radio communicationrealized by the above described configuration.

When sending a data stream, the transmission apparatus decides a weightto be multiplied on the transmission signal for each transmission systemfirst (ST1010). Then, the transmission apparatus multiplies thetransmission signal by this weight (ST1020) and sends a data stream(ST1030).

The reception apparatus receives the data stream sent from the abovedescribed transmission apparatus (ST1040) and detects a data error(ST1050). When some error is detected, the reception apparatus generatesa NACK signal (ST1060) and sends the NACK signal to the above describedtransmission apparatus (ST1070).

The transmission apparatus which has detected the NACK signal notifiedfrom the reception apparatus (ST1080) changes the weight decided inST1010 for each transmission system (ST1090), multiplies it on thetransmission signal to be retransmitted (ST1100) and retransmits thissignal to the reception apparatus (ST1110).

Then, an example of weights multiplied on a transmission signal underthe control of the weight control section 112 will be explained usingFIG. 5A and FIG. 5B.

FIG. 5A illustrates weights to be multiplied on transmission signalsarranged for data stream #A(S_(A)) and data stream B(S_(B)). Forexample, at the time of initial transmission of data, stream #A ismultiplied by weight w_(α), for the transmission system on the antenna106-1 (hereinafter referred to as “antenna #1”) side and multiplied byweight w_(β) for the transmission system on the antenna 106-2(hereinafter referred to as “antenna #2”). Furthermore, stream #B ismultiplied by weight w_(δ) for the transmission system on the antenna #1side and multiplied by weight w_(β) for the transmission system on theantenna #2 side.

Then, at the first retransmission, stream #A is multiplied by weightw_(γ) for the transmission system on the antenna #1 side and multipliedby weight w_(δ) for the transmission system on the antenna #2 side.Furthermore, stream #B is multiplied by weight w_(α) for thetransmission system on the antenna #1 and multiplied by weight w_(β) forthe transmission system on the antenna #2 side.

That is, here, the weight used for the data stream #A at the time ofinitial transmission is used for the data stream #B for the firstretransmission. Furthermore, the weight used for the data stream #B atthe time of initial transmission is used for the data stream #A for thefirst retransmission. At the time of the initial transmission and firstretransmission, weights used for the stream #A and stream #B areswitched round.

At the second retransmission, weights used for the stream #A and stream#B are switched round and the same weights as those at the time ofinitial transmission are used. At the third retransmission, weights usedfor the stream #A and stream #B are further switched round and the sameweights as those at the first retransmission are used. That is, everytime retransmission is repeated, weights used for the stream #A. andstream #B are switched round.

Here, weights to be multiplied on transmission signals arranged for theantenna #1 and antenna #2 from the different standpoint are shown inFIG. 5B. That is, this figure shows what signals are actually sent fromthe antenna #1 and antenna #2. The contents shown in FIG. 5A and FIG. 5Bare substantially the same.

Then, effects resulting from changes of weights between the initialtransmission and retransmission will be explained. When a weight ismultiplied on a transmission signal, the transmission signal acquiresdirectivity. However, this directivity does not mean that thetransmission signal is actually propagating in a specific direction asseen in the formation of beams by an array antenna technology, but it isa matter of mathematical expressions. This is because while there is alow fading correlation between antennas in the MIMO communication, thereis a high fading correlation between antennas in the array antennatechnology. However, this expression is used in studying effects ofweight multiplication of this embodiment because explaining with animage that a transmission signal is actually propagating withdirectivity facilitates an understanding.

FIG. 6 is a conceptual view illustrating directivity of a transmissionsignal. Signals sent from a radio transmission apparatus 100 usingweights w_(α) and w_(β) o follow a path #1 represented by a thick line,are reflected at some midpoint by a building 191 and reach a radioreception apparatus 150 without any significant reduction of theirintensity. On the other hand, signals sent using weights w_(γ) and w_(δ)follow a path#2 represented by a thin line, are reflected at somemidpoint by a building 192 and reach the radio reception apparatus 150affected by the propagation path with their intensity significantlyweakened.

At the time of initial transmission, the stream #A multiplied by weightsw_(α) and w_(β) follows the path #1 and the stream #B multiplied byweights w_(α) and w_(β) follows the path #2. Then, at the time ofretransmission the weights to be multiplied on the transmission signalsare switched round, and therefore the stream #A follows the path #2 andthe stream #B follows the path #1.

FIG. 7A, FIG. 7B, FIG. 8A and FIG. 8B show quality comparisons betweensignals sent from a conventional radio transmission apparatus andsignals sent from the radio transmission apparatus according to thisembodiment. These figures show conceptual views illustrating the quality(bar graphs) of received signals after being combined on the receivingside and a level L1 at which data can be received correctly.

FIG. 7A and FIG. 7B show cases of the conventional radio transmissionapparatus. FIG. 7A illustrates reception quality at the time of initialtransmission and FIG. 7B illustrates reception quality at the time ofdata retransmission.

In FIG. 7A, the quality level of the received signal of none of thestream #A and the stream #B exceeds the level L1. At this time, thereceiving side cannot receive data correctly, and therefore sends back aNACK signal to the transmitting side and the transmitting sideretransmits the data. However, when the time variation of thepropagation path environment is small, no significant improvement of thereception quality on the receiving side can be expected even at the timeof data retransmission. Thus, as shown in FIG. 7B, by combining thereceived signal at the time of initial transmission and the receivedsignal at the time of data retransmission after the data retransmission,the reception quality after the combination of the stream #A which hadoriginal reception quality at a level close to the level L1 at the timeof initial transmission exceeds the level L1. On the other hand, even ifdata is retransmitted, the reception quality of the stream #B after thecombination cannot exceed the level L1. Thus, the receiving side sendsback an NACK signal to the transmitting side several times more untilthe reception quality of the stream #B exceeds the level L1 and thetransmitting side retransmits data every time the NACK signal is sent.

FIG. 8A and FIG. 8B show the case of the radio transmission apparatusaccording to this embodiment. FIG. 8A illustrates reception quality atthe time of initial data transmission and FIG. 8B illustrates receptionquality at the time of data retransmission.

FIG. 8A is the same as FIG. 7A. None of the reception quality levels ofthe stream #A and stream #B exceeds the level L1. However, at the timeof retransmission, weights used for the stream #A and stream #B areswitched round, and therefore the environments of propagation pathswhich the stream #A and stream #B follow are placed in a condition as ifthose environments are averaged. In this way, both the reception qualitylevels of the streams #A and #B after combining the received signals atthe time of initial transmission and at the time of retransmissionexceed the level L1, and it is therefore possible to receive signalscorrectly.

In the above described configuration, the weight control section 112 ofthe radio reception apparatus according to this embodiment changesweights to be multiplied on the transmission signal from the weightsused for the previous transmission every time data is retransmitted.

This causes each data stream to be sent to the receiving side throughdifferent propagation path environments between the previoustransmission and retransmission, and therefore the probability that thesame data may contain errors successively is reduced and as a result,the data error rate characteristic after packet combination improves. Inother words, a diversity gain when the retransmission data is combinedon the receiving side increases and the reception performance on thereceiving side improves.

Furthermore, the weight control section 112 of the radio receptionapparatus according to this embodiment switches round the weightscorresponding to the respective antennas at the time of the previoustransmission and multiplies the transmission signals by those weightsevery time data is retransmitted.

For example, as shown in FIG. 7A or FIG. 8A, even if the receiving sidesends back a NACK signal at the time of initial transmission, receptionquality of all data streams is not averagely bad but it is often thecase that reception quality of only some data streams is bad.

At this time, by switching round weights to be multiplied on thetransmission signals between previous transmission and retransmission,the propagation path environments for the respective data streams areswitched round and averaged, and therefore the reception qualityimproves at an early stage.

Furthermore, since the weights already used at the time of the previoustransmission are reused at the time of retransmission by only switchinground the signals to be multiplied, and therefore there is no need forprocessing such as feeding back other information, for example, thepropagation path information detected on the receiving side to thetransmitting side.

Thus, according to this embodiment, weights to be multiplied ontransmission signals are switched round between the initial transmissionand retransmission, and therefore it is possible to increase a diversitygain obtained through data retransmission and improve receptionperformance on the receiving side.

Note that here a case where transmission data consists of two streams #Aand #B has been explained as an example, but the number of data streamscan be three or more, and in this case, it is possible to use weights tobe used at the time of data retransmission in rotation every time datais retransmitted. That is, in the case of three data streams, theweights used at the time of the initial transmission are reused at thethird retransmission.

Furthermore, here, a MIMO transmission having a low fading correlationbetween antennas has been explained, but it is also possible to use anarray antenna having a high fading correlation between antennas. At thistime, antennas are arranged so that the fading correlation betweenantennas becomes substantially 1. Directivity patterns as shown in FIG.6 are formed by multiplying transmission signals by weights. The stream#A multiplied by weights w_(α) and w_(β) at the time of the initialtransmission follows the path #1 and the stream #B multiplied by weightsw_(α) and w_(β) follows the path #2. Then, at the time ofretransmission, weights to be multiplied on the transmission signals areswitched round, and therefore the stream #A follows the path #2 and thestream #B follows the path #1. This makes it possible to averagepropagation path environments as with the above described case.

Here, a case where weights of the stream #A and stream #B are switchedround at the time of data retransmission has been explained as anexample, but it is also possible to select not the weights used at thetime of the previous transmission but totally different values asweights for retransmission.

Furthermore, here a case where the propagation path information, etc.,detected on the receiving side is not fed back to the transmitting sidehas been explained as an example, but for a radio transmission apparatusin which channel quality information is fed back from the receiving sideto the transmitting side for the purpose of increasing the channelcapacity and weights are decided so that more power is assigned tochannels of good quality, it is also possible to fine-tune weights basedon this fed back channel quality information and assign weights so thatthe channel quality exceeds the minimum level L1 in all paths.

Embodiment 2

FIG. 9 is a block diagram showing a configuration of a radiotransmission apparatus according to Embodiment 2 of the presentinvention. This radio transmission apparatus has the same basicconfiguration as that of the radio transmission apparatus shown in FIG.2 and the same components are assigned the same reference numerals andexplanations thereof will be omitted.

Features of this embodiment include that it is provided with IFFTsections 201, delay sections 202 and a delay control section 203, itcarries out a communication based on an OFDM scheme, adds delay timeswhich differ from one transmission data stream to another, transmits thedata streams through their respective antennas at different transmissiontimings at the time of transmission and thereby drastically changes thecharacteristic of a received signal on the frequency axis. Therefore, itis possible to drastically change the propagation path environment everytime retransmission is performed. Furthermore, transforming data into amulticarrier according to the OFDM scheme makes it possible to multiplexdelay signals with different delay times and send the multiplexedsignals.

Then, an operation of the radio transmission apparatus in the abovedescribed configuration will be explained.

The modulation section 102-1 carries out modulation processing on a datastream output from the buffer 101-1 and outputs the data stream. Thesignal output from the modulation section 102-1 is branched at somemidpoint and output to the IFFT sections 201-1 and 201-2. Likewise, themodulation section 102-2 carries out modulation processing on a datastream output from the buffer 101-2 and outputs the data stream. Thesignal output from the modulation section 102-2 is branched at somemidpoint and output to the IFFT sections 201-3 and 201-4.

On the other hand, the delay control section 203 includes a tablestoring a plurality of types of delay times, selects a delay time foreach transmission signal from the table according to a retransmissioncount notified from the retransmission count detection section 110 andoutputs the delay time to the delay sections 202-1 to 202-4. The delaytimes output from the delay control section 203 will be explained inmore detail later.

The delay section 202-1 delays the transmission timing of the signaloutput from the IFFT section 201-1 by a delay time output from the delaycontrol section 203 and outputs the delayed signal to the additionsection 104-1. Furthermore, the delay section 202-2 delays thetransmission timing of the signal output from the IFFT section 201-2 bya delay time output from the delay control section 203 and outputs thedelayed signal to the addition section 104-2.

Likewise, the delay section 202-3 delays the transmission timing of thesignal output from the IFFT section 201-3 by a delay time output fromthe delay control section 203 and outputs the delayed signal to theaddition section 104-1. Furthermore, the delay section 202-4 delays thetransmission timing of the signal output from the IFFT section 201-4 bya delay time output from the delay control section 203 and outputs thedelayed signal to the addition section 104-2.

The addition section 104-1 adds up the signals output from the delaysections 202-1 and 202-3, the transmission timings of which have beendelayed and outputs the addition result to the transmission radiosection 105-1. Likewise, the addition section 104-2 adds up the signalsoutput from the delay sections 202-2 and 202-4, the transmission timingsof which have been delayed and outputs the addition result to thetransmission radio section 105-2. The processing thereafter is the sameas that in Embodiment 1.

Then, an example of a delay time added to a transmission signal of eachsystem will be explained using FIG. 10A and FIG. 10B.

FIG. 10A shows delay times of transmission timings of transmissionsignals arranged for data stream #A(S_(A)) and data stream #B(S_(B)).For example, at the time of initial data transmission, the delay time ofthe stream #A is 0 for the transmission system on the antenna #1 sideand τ_(α) for the transmission system on the antenna #2 side.Furthermore, the delay time of the stream #B is 0 for the transmissionsystem on the antenna #1 side and τ_(β) for the transmission system onthe antenna #2 side.

Then, at the first retransmission, the delay time of the stream #A is 0for the transmission system on the antenna #1 side and τ_(β) for thetransmission system on the antenna #2 side. Furthermore, the delay timeof the stream #B is 0 for the transmission system on the antenna #1 sideand τ_(α) for the transmission system on the antenna #2 side.

That is, the combination of delay times of the data stream #A at thetime of the initial transmission is applied to the data stream #B at thetime of the first retransmission. Furthermore, the combination of delaytimes used for the data stream #B at the time of the initialtransmission is applied to the data stream #A at the firstretransmission. The combinations of delay times used for the stream #Aand stream #B at the time of the initial transmission are switched roundat the time of the first retransmission.

At the time of the second retransmission, the delay time used for thetransmission system on the antenna #1 side and the delay time used forthe transmission system on the antenna #2 side at the time of theinitial transmission are switched round. That is, the delay time of thestream #A is τ_(α) for the transmission system on the antenna #1 sideand 0 for the transmission system on the antenna #2 side, and the delaytime of the stream #B is τ_(β) for the transmission system on theantenna #1 side and 0 for the transmission system on the antenna #2side. At the third retransmission, the delay times used for the stream#A and stream #B are further switched round.

Now, by changing the standpoint, delay times of transmission signalsarranged for the antenna #1 and antenna #2 are shown in FIG. 10B. Thatis, this figure shows how signals are actually delayed and sent throughthe antenna #1 and antenna #2. The contents shown in FIG. 10A and FIG.10B are substantially the same.

FIG. 11 shows transmission timings of signals transmitted from the abovedescribed radio transmission apparatus on the time axis. Note that theactual transmission data streams are weighted (e.g., the signalstransmitted from the antenna #1 are w_(α)S_(A)+w_(γ)S_(B) as describedabove), but the same weights at the time of the initial transmission arealso used at the time of the retransmission in this embodiment, andtherefore notations of weights will be omitted to make it easier todistinguish one signal from another.

This figure shows that initial data transmission is performed at time 0and data retransmission is performed at time t1 after a lapse of acertain time. From the antenna #1, data stream #A (S_(A), S′_(A)) anddata stream #B (S_(B), S′_(B)) are transmitted at the same timing forboth the initial transmission and retransmission. On the other hand,from the antenna #2, S_(A) and S_(B) are transmitted with delay timesτ_(α) and τ_(β), respectively at the time of initial transmission, whileS′_(A) and S′_(B) are transmitted with delay times τ_(β) and τ_(α),respectively at the time of retransmission.

Here, when attention is focused on the data stream #A(S_(A)) at the timeof the initial transmission, S_(A) is transmitted with a time differenceτ_(α) provided between the antenna #1 and antenna #2. This is intendedto reduce a fading correlation between the antennas. On the other hand,when attention is focused on the data stream #A(S′_(A)) at the time ofthe retransmission, the time difference provided between the antenna #1and antenna #2 is τ_(β). That is, transmission is carried out in such away that the fading correlation between the antennas is changed at thetime of the initial transmission and at the time of the retransmission.The also applies to the data stream #B(S_(B), S′_(B)) in like fashion,

Furthermore, when attention is focused on both S_(A) and S_(B)transmitted from the antenna #2 at the time of the initial transmission,S_(B) is transmitted (τ_(β)−τ_(α)) behind S_(A). On the other hand, atthe time of the retransmission, S′_(A) is transmitted (τ_(β)−τ_(α))behind S′_(B) on the contrary. This means that transmission is carriedout in such a way that the difference in transmission timings betweenthe data stream #A and data stream #B (including the relationship as towhich is ahead and which is behind) is changed at the time of theinitial transmission and at the time of the retransmission. Here, forsimplicity of explanation, attention has been focused only on the datastream transmitted from the antenna #2, but the same thing is alsoapplied when the data stream transmitted from the antenna #1 is takeninto consideration together.

The effect of changing the difference in transmission timings betweendata streams at the time of the initial transmission and at the time ofthe retransmission will be explained below. Speaking plainly, frequencyselective fading is a result of signals with the same frequency and aphase difference of 180 degrees weakening each other. Thus, byintentionally delaying transmission timings of transmission signals,that is, shifting phases, it is possible to change a fadingcharacteristic that a signal receives. FIG. 12A to FIG. 12C showreception power at a conventional apparatus. A stream #A keeps a highreception level both at the time of initial transmission (see FIG. 12A)and at the time of retransmission (see FIG. 12B), while a stream #Bkeeps a low reception level both at the time of initial transmission andat the time of retransmission. Therefore, even if the data at the timeof the initial transmission and the data at the time of theretransmission are combined (see FIG. 12C), only the stream #A reachesthe level for correct reception and the stream #B remains at levelswhere it cannot be received correctly.

FIG. 13A to FIG. 13C show reception power according to this embodiment.The levels of received signals are affected by frequency selectivefading which differs between the streams. Therefore, the pattern of thereception level on the frequency axis differs between the streams. Atthe time of initial transmission (see FIG. 13A) and at the time ofretransmission (see FIG. 13B), delay processes applied to the stream #Aand stream #B are switched round, and therefore the levels of signalsafter being combined on the receiving side are averaged (see FIG. 13C)and both the stream #A and stream #B can be demodulated correctly.

Here, the figures describe a situation in which the phase of the fadingcharacteristic curve on the frequency axis is shifted by 180 degrees bydelaying transmission signals. However, it will be sufficient to changethe fading characteristic only to a certain degree by delayingtransmission signals in this embodiment, and the phase need not alwaysbe shifted by 180 degrees. Note that the fluctuation pitch of the fadingcharacteristic depends on the moving speed of a mobile station and thefrequency band used for communications, and therefore it is possible tocalculate a delay time for shifting the phase of the fadingcharacteristic curve by 180 degrees.

Furthermore, when the propagation path information such as channelquality is fed back from the receiving side to the transmitting side inthe above described configuration as in the case of Embodiment 1, it isalso possible to fine-tune delay times based on this fed backpropagation path information.

Thus, according to this embodiment, variations in the reception levelare averaged on the frequency axis of each stream for everyretransmission, and therefore it is possible to demodulate the combinedsignal correctly and improve the reception performance on the receivingside.

Note that Embodiment 1 and Embodiment 2 can be used in combination. Thatis, it is possible to further add a delay time to a transmission signalmultiplied by a weight to delay the transmission timing and transmit thedelayed signal. At this time, the effects of the respective embodimentsare superimposed, making it possible to further improve the receptionperformance.

As described above, the present invention allows a radio communicationsystem which transmits a plurality of data streams using a plurality ofantennas (e.g., adaptive array antenna technology, MIMO technology, STCtechnology, etc.) to increase, for example, when HARQ is applied, adiversity gain at the time of data retransmission and improve thereception performance on the receiving side.

This application is based on the Japanese Patent Application No.2002-268968 filed on Sep. 13, 2002, the entire content of which isexpressly incorporated by reference herein.

INDUSTRIAL APPLICABILITY

The present invention is applicable to a radio transmission apparatusand radio transmission method to which an adaptive array antennatechnology, MIMO technology or STC technology, etc., is applied.

The invention claimed is:
 1. A reception apparatus comprising: areceiver configured to receive, via a plurality of antennas, a pluralityof streams multiplied with a plurality of weights by a transmissionapparatus; an acknowledgment/negative acknowledgment (ACK/NACK)generator configured to generate ACK/NACK information based on an errordetected in the plurality of streams; a channel quality information(CQI) generator configured to generate CQI indicative of propagationpath quality of the plurality of streams; and a transmitter configuredto transmit the ACK/NACK information and the CQI to the transmissionapparatus, wherein, in case the NACK information is generated to requireretransmission from the transmission apparatus, the plurality of weightsto be multiplied with the plurality of streams, respectively, by thetransmission apparatus are rearranged between an initial transmissionand the retransmission by the transmission apparatus based on the CQIreceived from the reception apparatus.
 2. The reception apparatusaccording to claim 1, wherein the plurality of weights are rearrangedsuch that, in an even-numbered transmission, weights used in anodd-numbered transmission are used as weights for different streams inthe even-numbered transmission from streams used in the odd-numberedtransmission.
 3. The reception apparatus according to claim 1, wherein,in an even-numbered transmission, the receiver re-receives the same dataas data of an odd-numbered transmission.
 4. The reception apparatusaccording to claim 1, wherein a first weight multiplied with a firststream in an even-numbered transmission is the same as a second weightmultiplied with a second stream in an odd-numbered transmission.
 5. Thereception apparatus according to claim 1, wherein weights used in aneven-numbered transmission are selected from among weights used in anodd-numbered transmission.
 6. A reception method performed by areception apparatus, the reception method comprising: receiving, via aplurality of antennas, a plurality of streams multiplied with aplurality of weights by a transmission apparatus; generatingacknowledgement/negative acknowledgement (ACK/NACK) information based onan error detected in the plurality of streams; generating channelquality information (CQI) indicative of propagation path quality of theplurality of streams; and transmitting the ACK/NACK information and theCQI to the transmission apparatus, wherein, in case the NACK informationis generated to require retransmission from the transmission apparatus,the plurality of weights to be multiplied with the plurality of streams,respectively, by the transmission apparatus are rearranged between aninitial transmission and the retransmission by the transmissionapparatus based on the CQI received from the reception apparatus.